Ultra-deepwater floating platform

ABSTRACT

A floating platform system includes a hull design configuration ( 40, 50 ) for limiting maximum tendon loads and aiding in inhibiting resonant responses in the platform system leading to better motions for personnel, equipment and riser support, and to lighter and lower cost tendon systems.

BACKGROUND OF THE DISCLOSURE

The present invention relates to offshore floating platforms, moreparticularly to tension leg platforms (TLP) in ultra-deepwater, i.e.,8,000-10,000 ft water depth.

TLPs are floating platforms that are held in place in the ocean by meansof vertical structural mooring elements called tendons, which aretypically fabricated from high strength, high quality steel tubulars,and include articulated connections on the top and bottom (tendonconnectors) that reduce bending moments and stresses in the tendonsystem. Many factors must be taken into account during the design of thetendon system to keep the TLP safely in place including: (a) limitationof stresses developed in the tendons during extreme storm events andwhile the TLP is operating in damaged conditions; (b) avoidance of anyslackening of tendons and subsequent snap loading or disconnect oftendons as wave troughs and crests pass the TLP hull; (c) allowance forfatigue damage which occurs as a result of the stress cycles in thetendons system throughout its service life; (d) limiting naturalresonance (heave, pitch, roll) motions of the TLP to ensure adequatefunctional support for personnel, equipment, and risers; and (e)limiting vibrations in the platform system arising from vortex-inducedvibrations.

TLPs have been noted in the past to be water depth limited to waterdepths of 3000′, or 4000′, or 5000′, or 6000′, depending on when and whois asked. The primary limitation in extending the limits for TLPapplications has been the cost and weight penalty for maintaining tendonstiffness to prevent natural periods of heave/pitch/roll from becominglonger than the commonly accepted 2-4 seconds. Keeping these responseperiods short prevents them from being excited at resonance by direct(first order) wave energy. In order to maintain the same stiffness asshallower depth systems, a tendon must be increased in area by a ratiosimilar to the ratio of length increase. In simple terms, the tendonmass increases as the third power of the water depth. As the tendon massincreases in increasing water depths, the tendon also adds to the systemprimary mass for heave/pitch/roll modes, and requires additionalstiffness to maintain the same modal periods. As a consequence, thetraditional approach to TLPs is limited by increased cost, and bydecreased payload, with increasing water depth, the limit depending onthe levels of optimization employed and the cost sensitivity of theapplication.

It is therefore an object of the present invention to provide a floatingplatform system including a hull design to limit maximum tendon loadsand aid in inhibiting resonant responses in the platform system leadingto better motions for personnel, equipment and riser support, and tolighter and lower cost tendon systems.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained can be understood indetail, a more particular description of the invention brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

It is noted, however, that the appended drawings illustrate only typicalembodiments of this invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1 is a side view of a mono-column floating platform;

FIG. 2 is a partial perspective view of a conventional four-columnfloating platform;

FIG. 3 is a partial perspective view of a four-column floating platformof the present invention;

FIG. 4 is a partial perspective view of an alternate embodiment of afour-column floating platform of the present invention; and

FIG. 5 is a table of tension responses for different floating platformconfigurations.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIG. 1, a mono-column floating platform generallyidentified by the reference numeral 10 is shown. The floating platform10 includes a column or hull member 12 projecting above the watersurface 14 supporting a platform deck 16 thereon. Pontoons 18 extendradially outward from the base of the hull 12. The floating platform 10is anchored to the seabottom 20 by tendons 22.

In a typical tendon design, steel tendons are utilized to secure thefloating platform to the seabottom. As exploration and production of oilreserves expand into deeper waters, the design of the tendon systembecomes more critical and begins to dominate the platform costs. Thetendon system must be designed to operate between tolerable minimum andmaximum tensions, to restrict natural resonance motions, and to limitthe fatigue damage caused by each stress cycle. The latter two aretypically accomplished by increasing the cross-sectional area of thesteel tendon, which increases the tendon axial stiffness. But thisincreases the weight of the tendon and reduces the payload carryingcapacity of the floating platform.

In accordance with the present invention, reducing tension response isaccomplished by changing the hull form or configuration for theultra-deepwater platform installations. A mono-column TLP configurationis shown in FIG. 1. Alternate configurations are shown in FIGS. 2-4. TheTLP 10 of FIG. 1 (and payload) is located in a water depth of 8500 ft.The axial stiffness of the tendons 22 is taken to be 300 kips/ft pertendon. The alternate platform configurations of FIGS. 2-4 includevariations in hull geometry with a number of constraints to providecomparable cases to the reference TLP 10. Each alternate platformconfiguration has the same hull weight, gyradii, payload anddisplacement as the reference TLP 10. The location of the tendon porchis allowed to vary by configuration. It is assumed that the porches arelocated at the tips of the pontoons 18; pontoon lengths may vary betweenthe configurations.

Referring now to FIG. 2, a four-column conventional TLP platform 30 isshown. The platform 30 includes four columns 32, one at each corner,interconnected by horizontal members 34. The columns 32 project abovethe water surface and support a platform deck thereon. The platform 30is anchored to the seabottom by tendons (not shown in the drawings), twoat each column 32. The draft and column/pontoon ratio are similar to theplatform 10. In a performance evaluation based on the tendon tensionresponse, the platform 30 performed better than the platform 10, but notas good as the platform configurations shown in FIGS. 3 and 4. Also, animportant consideration with the platform 30 is the four corner squareshape compared to the three sided configuration of platform 10. For acomparable design, platform 30 will have 33% more tendons, in thepresent instance eight instead of six, which will have a substantialcost impact in 8500 ft water depth.

Referring now to FIG. 3, a platform 40 includes three columns 42 locatedat or near the distal ends of each pontoon 18. Increasing the size ofthese outer columns 42, while maintaining the same water plane area(reducing the central column 12), typically results in an increase ofheave response and a decrease in roll/pitch response over most of thewave frequency range. Simulations analysis indicate that tension RMSvalues under fatigue sea states are best when the outer column 42 isless than half the diameter of the central column 12. However, forhurricane sea states, the tension RMS values are best when the outercolumn 42 is approximately the same diameter as the central column 12.Overall, the tension RAO of the platform 40 is significantly less thanthat of the mono-column platform 10 or of the conventional 4 columnplatform 30.

Referring to FIG. 4, a platform 50 includes three battered outer columns52 located at or near the distal ends of each pontoon 18. The slightbatter of the outer-columns 52 substantially reduce the tension RMSunder fatigue conditions, while the tension RMS under hurricaneconditions is barely affected. There appears to be an optimum batterangle of less than 10 degrees, with a value of 6 to 8 degrees as moretypical. The optimum angle appears to be dependent on the volumetricratio of pontoon and column.

The performance evaluation of each TLP configuration, summarized in FIG.5, is based on the tendon tension response. An estimate of the tensionRMS, an indicator of fatigue damage and extreme loads, is computed for afatigue sea state and for a 100-year hurricane sea state. The comparisonis based on relative performance of fatigue sea state results, andrelative performance of hurricane sea state results.

Some of the findings are expected, for example the effect of deeperdraft and longer pontoons is part of the consideration given in currentdesign practices. In system design, these are partially balanced byincreases in cost of the hull to achieve these improvements.

The addition of the three end-of-pontoon columns 42, 52 takes thetraditional mono-column triangular shape and changes it from amono-column design to a multi-column design. As has been seen insemi-submersible design, the phase differences between loads onmulti-column structures produce cancellation between columns andresultant improvements in motions and total loads. The disadvantage isgenerally greater internal racking and squeeze/pry loadings into thestructure itself. Also, for the mono-column platform, the substantialchange between a deck supported on a single column and one supportedbetween four columns, with the introduction of hull loads into the deck,produces a substantial change in the way the deck will have to bedesigned and analyzed.

The battered columns configuration was something of a surprise. Theoriginal reason for inclusion of battered columns was to provide abetter load path for support of the deck. In evaluating the results,however, the improvement due to the battered columns 52 appears to bedue to the fact that the inclination gives the columns 52 pontoon-likeproperties. The portion of the columns 52 that is not under the shadowof the surface water plane has water acting both above and below,whereas the portion of the column 52 that is under the shadow of thesurface water plane has water acting only from below. As a result, it ispossible to modify the balance between surface piercing and non-surfacepiercing buoyancy without changing the actual dimensions of the pontoonsand columns. Since designers are typically limited structurally to theamount of displacement that they can allocate to the pontoons withoutthe column getting structurally too “skinny”, especially in deep draftconfigurations, battering the columns enables better optimization of thepontoon/column.

Referring still to FIG. 5, it will be observed that the four-columnconventional TLP 30 has better performance per tendon than themono-column TLP 10 and a deep draft mono-column TLP, but this istempered by the need for two additional tendons 22 for the TLP 30.However, it is clear that the four-column TLP 40 has better performancethan the conventional TLP 10, especially in the fatigue sea states. Inultra-deep water, the cost of providing and installing additionaltendons and piles is likely to give the four-column TLP 40 even moreadvantage over a conventional TLP 10.

In conclusion, the results summarized in FIG. 5 give substantialguidance in optimizing the platform hull form for ultra-deep waterinstallations. The addition of multi-column configurations will open theopportunity for free floating stability, including possible majorchanges in installation costs over the crane assisted ballasting andoffshore deck installation currently used. The use of the triangularbase and radiating pontoons keeps some of the benefits of themono-column TLP (fewer tendons, constraint of the risers at the keel,simple geometry to increase baseline), while substantially improving theperformance and costs in ultra-deep water.

While a preferred embodiment of the invention has been shown anddescribed, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims which follow.

1. A floating platform, comprising: a) a central column having an upperend and a lower end; b) pontoons extending radially outward from saidlower end of said central column; c) outer columns located at the distalends of said pontoons, said outer columns extending substantiallyvertically; d) a deck supported above the water surface on said centralcolumn and said outer columns; and e) anchor means securing saidfloating platform to the seabottom.
 2. The floating platform of claim 1wherein said outer columns are battered.
 3. The floating platform ofclaim 2 wherein the batter angle is less than 10 degrees.
 4. Thefloating platform of claim 3 wherein the batter angle is in the range of6 to 8 degrees.
 5. The floating platform of claim 1 wherein said outercolumns are located along the pontoons but not at the distal ends ofsaid pontoons.
 6. The floating platform of claim 5 wherein said outercolumns are battered.
 7. The floating platform of claim 6 wherein thebatter angle is less than 10 degrees.
 8. The floating platform of claim7 wherein the batter angle is in the range of 6 to 8 degrees.
 9. Amethod of minimizing floating platform tendon tension response inultra-deep water comprising installing said platform in a body of waterand anchoring said platform to the seabottom, wherein said platformcomprises: a) a central column having an upper end and a lower end; b)pontoons extending radially outward from said lower end of said centralcolumn; c) outer columns located at or near the distal ends of saidpontoons, said outer columns extending substantially vertically; and d)a deck supported above the water surface on said central column and saidouter columns.
 10. A floating platform, comprising: a) four cornercolumns each having an upper end and a lower end; b) pontoons extendingbetween said lower ends of said corner columns forming a square orrectangle; c) the four corner columns being battered such that the topsare closer together than the bottoms; d) a deck supported above thewater surface on said corner columns; and e) anchor means securing saidfloating platform to the seabottom.